This invention relates to a Stirling engine of the type receiving heat inputs from a remote source, and particularly to a heat pipe transfer tube connected to such an engine.
In one form of the Stirling cycle engine, a number of reciprocating pistons within cylinders are arranged in generally parallel relationship in a square cluster. The top of each cylinder is attached to a gas duct which connects to a cylindrical column having a heat exchanger, regenerator, and cooler stacked end-to-end. One means of providing heat input energy to such a Stirling engine is to employ a heat pipe which has a remotely situated evaporator which absorbs heat from some source such as solar energy, combustion flue gasses, etc., which cause the working fluid to vaporize. The vaporized working fluid is transported to the engine heat exchanger where it condenses, thus giving up its latent heat of evaporation, and then returns to the heat pipe evaporator.
In such devices according to the prior art designs, a number of shortcomings exist in the design of the heat pipe conduits which transfer the working fluid from the evaporator to the engine heat exchanger (i.e., condenser). Since the working fluid vapor and liquid phases are typically transferred within a single conduit and travel in opposite directions, the liquid working fluid can become entrained within the vapor, particularly when the engine is operating at a high power setting. Such entrainment reduces the heat transfer rate to the engine and further can prevent adequate liquid working fluid return to the heat pipe evaporator which can lead to localized areas of the evaporator "drying out" and becoming excessively heated, potentially leading to mechanical failure. Furthermore, since Stirling engine heat exchangers are very compact, condensed heat pipe working fluid tends to collect in the heat exchanger due to capillary action which represents a waste of a certain volume of the working fluid, and also decreases the useful surface area in the heat exchanger. In view of the foregoing, there is a need to provide an improved heat pipe conduit and a means for reducing the retained volume of liquid working fluid within the Stirling engine heat exchanger.
During initial startup of a Stirling cycle engine, contaminant gases tend to be present in the heat pipe which supplies heat inputs to the engine. Such gases are present due to outgasing from the engine and heat pipe components and from other sources. Since the flow direction of the vaporized heat pipe working fluid is toward the condenser, the contaminant gases tend to collect in the upper portions of the heat pipe assembly in the area of the condenser. Such gases can form a "plug" which prevents the heat pipe working fluid from contacting the engine heat exchanger thus interfering with heat inputs to the engine. The use of getters using various substances fro absorbing impurity gases is known. The elements lanthanum and calcium are capable of absorbing many impurity gases if they are brought to an elevated temperature, e.g., 600 to 800 degree C. Such getters need to communicate with the impurity gases collecting in the upper portions of the engine heat exchanger. During initial operation, however, the heated heat pipe working fluid does not flow into these upper portions due to the previously mentioned impurity gas plug. Accordingly, there is a need to provide a getter assembly which absorbs impurity gases during initial Stirling engine startup.
The above mentioned desirable features are achieved in accordance with this invention through an ipmroved design heat pipe working fluid conduit assembly. The assembly features a shell and tube construction in which a flared shell joins the heat exchanger and provides a means of reducing the velocity of vaporized heat pipe working fluid as it enters the heat exchanger. This reduction in velocity tends to minimize problems of liquid entrainment within the vapor. As a further step to reduce entrainment, a separate liquid heat pipe working fluid return duct is provided within the conduit outer tube which provides isolation of the phases. A surface tension breaker is used which communicates the engine heat exchanger with the liquid return pipe as a means of reducing the volume of liquid working fluid retained by the heat exchanger. In another feature of this invention, a getter unit is provided adjacent to the condenser of the heat pipe which has an auxiliary heater for heating the active compounds of the getter, enabling it to absorb the impurity gases before heat pipe working fluid is capable of heating the getter assembly.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.